Postgresql installation problem

Hi

I have installed a 7.1.3 postgresql version on my linux server. (RedHat 7.2)

When I type "createuser xxxxxxxx" it asks me : Shall the new user allowed to create databases (y/n)?
Shall the new user allowed to create more new user (y/n)?

I answer yes to the both questions and then a bug report said :
" psql : connetDbstart() -- connect() failed - no such file or directory
Is the postmaster running locally and accepting connections on Unix socket :'/tmp/.s.PSQL.5432'
createuser : creation of user 'xxxxxxx' failed.

I already had answers to this but I am a newbie in linux, so i didn't understand what i have to do exactly to resolve this problem.

Someone wrote me :
1. Did you initialized your database with initdb ?
2. Have you started Postgres with postmaster -i or pg_ctl start ?
3. Have you modified parameters in $PGDATA/postgresql.conf file ?
Follow your answers, you have solved your problem

1.When i type initdb it said me that it's not possible to create specific directories (pgsql and data)
2. How am I supposed to use the postmaster or pg_ctl ??
3. The installation is not complete so i haven't a postgresql.conf file ....

Thanks for your help.
Pierre-Alexis